In every communications network one of the major problems to be solved is how to route the information in the best possible way between the source and the destination. Routing protocols may generally be classified into two different categories, namely centralised and distributed routing protocols. In centralised routing some central node determines the path to follow from source to destination, whereas in distributed routing all nodes in the network participate in the routing decision and each node is able to calculate a “good” path to follow from itself in the direction of the destination. Most currently used routing protocols are developed for wired networks, in which the shortest path is selected, and then used over some period in time. A typical metric for the shortest path may aim to minimize delay.
Routing in wireless networks has traditionally been based on adaptations of the methods developed for wired networks. This means that in the general case a path is selected and all packets transmitted between a source and a destination follow the same path, unless something major happens that changes the structure of the network. That is, in practice, single path routing is commonly used in wireless networks.
Wireless networks are, for several reasons, generally more dynamic than wired networks. The experienced interference levels at different receivers will vary depending on whether anything is being transmitted on a particular link at a given time and with what power. Moreover, propagation conditions are exposed to time variations depending on the user location and obstacles in the environment, i.e. movement of nodes as well as obstacles affects the channel. Similarly, the channel may vary in the frequency domain due to time dispersive propagation. Short term fading may also arise, due to large number of randomly scattered (and possibly direct) signals superimposed at the receiver antenna.
Since the channel fluctuates, not only does the desired received carrier fluctuate, but also the experienced interference will fluctuate. This is due to channel variations, but also to what appear as random and unpredictable traffic variations. The quality for a link is dependent both on the desired signal as well as the interference, and typically both vary.
Trying to follow the rapid changes in the network when routing packets is often infeasible since it may lead to extensive control signalling. One other option is to calculate the routing decisions on the basis of averages.
Thus, one problem of single path routing in wireless networks is that local and instantaneous propagation and queue conditions that can provide additional guidance in the forwarding decision are often not fully exploited. For example one node in a predetermined path may experience a temporary and high path loss due to fast fading, which is not reflected in the routing choice.
What has been described above is true for most routing methods, but some routing methods are opportunistic in the sense that they make use of variations in the network to select a transmission path that is particularly advantageous at a given moment. For example, multiuser diversity, as described in WO 2004/091155, can be used in opportunistic routing by transmitting at any given time to the user that at this particular time has a better connection than the others. Subsequently, the user forwards any data according to the same transmission principle, and the data heads towards the destination. By always transmitting on the paths that are momentarily good the network is utilized in the best possible way. Since the received signal quality will generally vary over time, all users will in the long run receive data.
In recent years nodes and terminals have emerged that can use two or more different radio access technologies, so called multi access or multi-radio access, each radio access technology being characterized by certain strengths and weaknesses for different conditions. One such combination that is often found is Wireless Local Area Network (WLAN) and Wideband Code Division Multiple Access (WCDMA). Typically different radio access technologies make use of separate frequency bands, meaning that multi access capable nodes and terminals can get access to multiple independent spectrum resources. In existing multi access terminals and nodes the source chooses which radio access technology to use at a given time, the decision being based on one or several factors such as current channel conditions, interference levels in the corresponding frequency bands, and/or more general properties such as typical quality of service level offered, or simply by availability of the respective radio access technologies at the current location.
A network in which only one single access technology is used is referred to as a homogeneous network, whereas a network allowing multiple access technologies is often referred to as a heterogeneous network. Traditionally, routing in homogenous networks has been the object of many studies, while the interest in wireless routing for heterogeneous networks has been significantly smaller. In particular, routing in heterogeneous networks has not addressed radio aspects yet. Moreover, existing routing schemes for heterogeneous networks are not responsive to the rate of changes experienced in a radio environment.